Ruby on Rails

Olá, tudo bem... sou nova em ROR e estou precisando de ajuda com o
active query...
Bom.. tenho 3 models..
Staff, Appointment e Service..
Gostaria de saber quanto que esse staff gerou de serviço..
Obs.. O Staff possui 2 models filhos (Employees e Freelancer)
GOstaria de saber com faço isso.. fiz um sql, mas não sei como coloco
isso no código
SELECT count(S.value)
FROM Staff SF INNER JOIN Appointment A ON (SF.id = A.staff_id)
INNER JOIN Service S ON (A.service_id = S.id)

Alguém pode me ajudar??

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